Carbon dioxide absorption using alkaline solutions has been described, in which carbon dioxide is absorbed into a solution of alkali-metal hydroxide. The resulting dissolved alkali-metal carbonate is treated (e.g., with calcium hydroxide) to generate a solid carbonate product (e.g., calcium carbonate) and the original alkali-metal hydroxide. The solid carbonate product is heated in a kiln to release carbon dioxide and generate a metal oxide. Upon dissolving the metal oxide back into water, the causticizing reactant (e.g., calcium hydroxide) is regenerated. These conventional caustic recovery processes include the use of calcium hydroxide, and alternative causticization processes include the use of borate (auto-causticization) and iron oxide or titanium dioxide (direct causticization). These causticization processes typically include heating to temperatures of at least 100° C., with the high temperature reactions in the kiln occurring around 900° C.
In the Benfield Process, CO2 is absorbed into a concentrated solution of potassium carbonate at a given temperature of usually around 70° C. The solution comes to equilibrium under these conditions, and then is heated further under an applied pressure to temperatures generally in excess of 130° C. in which equilibrium is again reached, this time at a higher partial pressure of CO2. The result is a system which absorbs CO2 at low temperature and desorbs it at high temperature. This type of process is generally used to absorb CO2 and other acid gases from high concentration flue gas streams.